We had an extra 5th and 6th this year from midseason trades of trash OL we were gonna have to cut anyways. Having a 9 rookie class is wholly unfeasible. Next year's 4th round pick will be covered by a compensatory 4th (which can go up to a 3rd) when Shaq Lawson signed with Miami today - and we weren't going to sign him, so that's a free pick imo.
I think you're discounting the idea that the Bills don't have time to develop a WR. They have a 1st round QB who they'll have to pay even if he's only Bortles-esque, an RB who is gonna be good, a top 2 CB who will need to be paid, and two cornerstone LBs who anchored the #2 defense in the league. Diggs is signed for another 4 years at $11m a year, which is a steal (though obviously he'll want to restructure, hopefully we can push that off a bit).
We have been hitting on late round picks, true. We have 4 starters of our 22 from round 5 or later, all on rookie contracts, and it jumps up to like, 6 with major bit players, but imo that means we were just ahead of the curve on hitting on those late picks and can afford to not pick a few more of them.
Bills also signed DE Mario Addison late night, continuing the Carolina-Buffalo connection. He's a bit old for my taste, but 9+ sacks a year for the past four years is okay to me. Still no contract details.
One final point: the Bills have no more excuses on offense. This is the year the final evaluation on Josh happens. If he can't hit one of the best route runners with some of the best separation in the league. We hit on our TE and RB in last year's draft, besides perhaps a RT we're set on the offense. There should be nothing else needed. The defense is elite. It's now or never, before we have to pay like the half of our team who are on rookie contracts.